A. Technical Field
The present invention relates to a process for treating a particulate water-absorbent resin. Particularly, the present invention relates to an improvement in a process for producing a particulate water-absorbent resin itself, which comprises a step of pulverizing a water-absorbent resin in part of the entire steps, in addition to an improvement in transporting or storing a particulate water-absorbent resin during or after producing the particulate water-absorbent resin. More particularly, the invention relates to processes for pulverizing the water-absorbent resin, and transporting and storing the particulate water-absorbent resin, with high accuracy and productivity. Examples of the water-absorbent resin include water-absorbent resins which are applied to wide uses, for example, as absorbing agents for sanitary materials (e.g. sanitary napkins and disposable diapers), or as water-holding agents and dehydrators in the agricultural and gardening field and the field of civil engineering works.
B. Background Art
A particulate water-absorbent resin is generally used as a powdery particulate product having a particle diameter of 1 millimeter to several tens micronmeters.
Therefore, the particulate water-absorbent resin is produced by drying a hydrogel polymer obtained by polymerizing hydrophilic monomers, and then pulverizing the resultant dry polymer with a pulverizer.
In the above-mentioned pulverization, a cohered matter of the particulate water-absorbent resin adhered inside the pulverizer or to a pipe near an exhaust outlet of the pulverizer. Therefore, it happened that: the pulverizing procedure could not be carried out stably because of causing clogs, and properties of the particulate water-absorbent resin became low because of contaminating the particulate water-absorbent resin with the cohered matter.
The particulate water-absorbent resin obtained as shown above, are changed into a product thereafter by way of a classifying step, a pulverizing step, a blending step for additives, and a surface-treating step. The respective steps are connected by machinery transporting apparatuses which needs little power, or by a pneumatic conveyer usually. The particulate water-absorbent resins also undergo a step of transporting even in a stage after producing the particulate water-absorbent resins, for example, when using the particulate water-absorbent resins. In this step of transporting during or after the production, the cohered matter of the particulate water-absorbent resins adhered to the transporting apparatus. Therefore, it happened that: the transporting procedure could not be carried out stably because of causing clogs, and properties of the particulate water-absorbent resins became low because of contaminating the particulate water-absorbent resins with the cohered matter in the same way as mentioned above.
There were similar problems as mentioned above when storing the particulate water-absorbent resins between the above-mentioned two steps during the production, and storing a final product after the production.
In a process for obtaining particulate water-absorbent resins, the particulate water-absorbent resins are often classified. Then, it happens that there was a similar problem as mentioned above. Therefore, the present inventors filed patent applications (JP-A-202187/1998 and EP 855232) concerning a process for classifying a particulate hydrophilic polymer and a sieving device. The process comprises a step of using the sieving device in a heated and/or thermally insulated state, and enables to classify the particulate hydrophilic polymer in particle size with high accuracy and productivity.
In addition, continuous agglomeration of an absorbent resin powder was described in U.S. Pat No. 5,369,148. In a method of carrying out the continuous agglomeration, a cylindrical member was heated in order to prevent the highly adhesive agglomerate, which has particles of the absorbent resin powder cohesively united with an aqueous liquid, adhering to the cylindrical member. On the other hand, the present invention as described below relates to processes for pulverizing the water-absorbent resin, and transporting and storing the particulate water-absorbent resin, wherein the aimed water-absorbent resin or particulate water-absorbent resin is not adhesive but free-flowing. It is impossible to anticipate that: heating which had effect on preventing the highly adhesive agglomerate adhering, also has effect on the water-absorbent resin or particulate water-absorbent resin which is not adhesive but free-flowing.
A. Object of the Invention
Thus, an object of the present invention is to provide a process for treating a particulate water-absorbent resin, which enables to carry out the above-mentioned pulverization stably in a process for producing the particulate water-absorbent resin, and the transportation or storage of the particulate water-absorbent resin stably during or after producing the particulate water-absorbent resin, and to prevent the contamination of the cohered matter of the particulate water-absorbent resin; and involves no or little deterioration of properties.
B. Disclosure of the Invention
The present inventors diligently studied about causes of forming the above-mentioned cohered matter. As a result, they found that: the water-absorbent resin as obtained by drying the hydrogel polymer has a water content of about 3 to about 15%, the pulverized particulate water-absorbent resin tends to be aggregated by the water heat-generated in the pulverization of the polymer, and the cohered matter adheres to an internal sidewall of the pulverizer, to parts for the pulverizer, or around the exhaust outlet of the pulverizer. In addition, they also found that: the cohered matter is formed by the water in the same way as mentioned above when the particulate water-absorbent resin having extremely enlarged surface areas by the pulverization is transported or stored.
Therefore, the present inventors found that the above-mentioned problems are solved by using a pulverizer, a transporting apparatus or a storage tank in a heated and/or thermally insulated state in order to inhibit the cohesion caused by the water content of the particulate water-absorbent resin. As a result, the present invention was attained.
Thus, a process for transporting a particulate water-absorbent resin, according to the present invention, comprises a step of transporting a particulate water-absorbent resin obtained by pulverizing a dry water-absorbent resin product, and is characterized by carrying out at least one selected from the group consisting of following (1) to (3) when transporting the particulate water-absorbent resin.
(1) heating at least one portion of a surface getting contact with the particulate water-absorbent resin from the outside,
(2) maintaining the temperature of at least one portion of a surface getting contact with the particulate water-absorbent resin at 30 to 150xc2x0 C., and
(3) maintaining the temperature of at least one portion of a surface getting contact with the particulate water-absorbent resin above a temperature that is lower than a temperature of the particulate water-absorbent resin by 20xc2x0 C.
A process for storing a particulate water-absorbent resin, according to the present invention, comprises a step of storing a particulate water-absorbent resin obtained by pulverizing a dry water-absorbent resin product, and is characterized by carrying out at least one selected from the group consisting of the above-mentioned (1) to (3) when storing the particulate water-absorbent resin.
A process for producing a particulate water-absorbent resin, according to the present invention, comprises a step of pulverizing a dry water-absorbent resin product in order to obtain a particulate water-absorbent resin in part of the entire steps, and is characterized by carrying out at least one selected from the group consisting of the above-mentioned (1) to (3) in the step of pulverizing.
In this way, the present inventors found that the fundamental process described in JP-A-202187/1998 (EP 855232) can be applied to the respective steps of pulverizing, transporting, and storing.
Surprisingly, the above improved processes for transporting and storing can preferably be applied to a particulate water-absorbent resin that is surface-crosslinked and has little water content.
These and other objects and the advantages of the present invention will be more fully apparent from the following detailed disclosure.